Heat is often created as a byproduct of industrial processes where flowing streams of liquids, solids or gasses that contain heat must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Sometimes the industrial process can use heat exchanger devices to capture the heat and recycle it back into the process via other process streams. Other times it is not feasible to capture and recycle this heat because it is either too high in temperature or it may contain insufficient mass flow. This heat is referred to as “waste” heat and is typically discharged directly into the environment or indirectly through a cooling medium, such as water.
Waste heat can be utilized by turbine generator systems that employ well-known thermodynamic methods, such as the Rankine cycle, to convert the heat into useful work. Typically, this method is a steam-based process where the waste heat is used to generate steam in a boiler in order to drive a turbine. The steam-based Rankine cycle, however, is not always practical because it requires heat source streams that are relatively high in temperature (e.g., 600° F. or higher) or are large in overall heat content. Moreover, the complexity of boiling water at multiple pressures/temperatures to capture heat at multiple temperature levels as the heat source stream is cooled, is costly in both equipment cost and operating labor. Consequently, the steam-based Rankine cycle is not a realistic option for streams of small flow rate and/or low temperature.
The organic Rankine cycle (ORC) addresses some of these issues by replacing water with a lower boiling-point fluid, such as a light hydrocarbon like propane or butane, or a HFC (e.g., R245fa) fluid. However, the boiling heat transfer restrictions remain, and new issues such as thermal instability, toxicity or flammability of the fluid are added.
There exists a need in the art for a system that can efficiently and effectively produce power from not only waste heat but also from a wide range of thermal sources.